Polyvinyl chloride (PVC) is the best known and most widely used of the vinyl plastics, accounting for about 30% of the plastics used worldwide. For many uses, the PVC has to be used in plasticized form. The plasticizer, usually a high-boiling liquid, is added by mixing with the PVC and preserves its flexibility. However, with time the plasticizer gradually escapes from the PVC object, rendering it brittle and breakable.
Various attempts have been made in the past to produce crosslinked PVC in order to prevent plasticizer migration. Amongst these there may be mentioned UV irradiation of a mixture of trimethylpropane trimethylacrylate with PVC, resulting in a crosslinked polymer which has thermal stability and improved abrasion resistance. Also compounds such as trialkylacrylates, triallylisocyanurate, allyl esters, divinylbenzene and triacrylates have been used as crosslinking agents. When such materials are added to the entire polymer, the crosslinking results in a brittle polymer which is difficult, if not impossible, to mold, and/or a product which is undesirably brittle.
Attempts have also been made to use a protective layer of epoxyacrylate as a surface layer to a PVC substrate, the epoxyacrylate being crosslinked by application of UV radiation. These attempts resulted in a layer having poor adhesion to the PVC substrate. A further attempt in this direction was disclosed recently in the patent U.S. No. 4,806,393, issued to the same applicant, which describes the coating of a plasticized PVC substrate with a thin layer of a cross-linkable thiocarbamate modified PVC and crosslinking by heating. Although this layer may prevent the leakage of plasticizers, the process has the disadvantages of requiring a special oven for the removal of the solvent used when applying the modified PVC on the substrate and for the crosslinking step. Other disadvantages of this process include the need of the preparation of the starting product in a preliminary step by reaction of PVC and the thiocarbamate, resulting in an unstable crosslinkable product which, on storage, may crosslink by itself. Besides, it is very difficult to control the temperature of the reaction. Above 45.degree. C., the material may crosslink by itself and will not dissolve in the solvent, thus making impossible its use for coating a PVC substrate.
Thin films, tubes or beads of size .gtoreq.0.1 .mu. made of PVC, plasticized PVC or copolymers of PVC are used as a means for controlled release of pesticides, fertilizers and even drugs into the surrounding environment. However, there are difficulties in this technique because in many cases the release is too fast, in which cases the beads have to be replaced very often and become less efficient.
These and other disadvantages of the prior art are overcome by the process of the invention.